The invention relates to a circuit for automatic compensation of transient overshoot, undershoot and delay in systems utilizing filter networks with reactive components.
Filter networks having reactive components are known to provide overshoot or undershoot transients in response to sudden D.C. signal level changes applied thereto. These transients, in turn, add to the delay in the filter response. Even in critically damped filters, which do not exhibit such overshoot or undershoot transients, a delay in response to sudden D.C. level changes occurs due to recovery time necessary for adjusting the reactive elements of the filter to a new D.C. signal level.
The above-indicated inherent features of reactive filter networks are particularly undesirable in applications where fast response, that is, minimum delay in filter operation, is required.
For example, in automatic control circuits utilizing servo loops with filters having reactive components, input signal variations must be followed by the loop accurately and with minimum delay. When a sudden input signal level change in the form of a D.C. voltage step is applied to such circuit, for example, to produce a position offset in a positioning servo loop, the reactive compounds charge or discharge to the new D.C. level, thus causing damped oscillations and added filter delay due to overshoot or undershoot transients.
Prior art servo circuits compensate for the above effects by reducing the gain of the servo loop to minimize oscillations. The result is a servo operating below the optimum loop gain and, thus, with reduced accuracy.
In some applications, for example, in high density magnetic recording and playback, reducing the gain of the servo loop is impractical, since it decreases positioning accuracy. To the contrary, in the latter applications, where both the recording track width and inter-track spacing is reduced, the accuracy of the head positioning servo must be further improved, and the servo response time minimized. To this effect, it is desirable to further increase the servo gain while obtaining stable operation in close proximity of mechanical resonant frequencies, for example, by utilizing reactive filter networks. However, reactive filters are known to increase the servo loop response time due to overshoot or undershoot transients provided in response to D.C. step signals injected in the loop.
An example of a prior art device comprising a positioning servo circuit is described in the Operation and Maintenance Manual, DM 940/DM 980 Disk Storage Drive, No. 3306658-01, issued October 1977, by Ampex Corporation, assignee of this patent application. More particularly, FIGS. 3-4 on page 3-9 of that Manual shows a basic servo block diagram, including a fine mode positioning servo control circuit. The latter circuit is utilized for maintaining the transducer assembly of the disk storage within a designated track location until a subsequent coarse positioning mode is initiated, as it is known in the art. As it is shown, for example, in the detailed circuit diagram No. A 04, sheet 1 of 3, and indicated on page 3-57, paragraphs 6 and 7, of the Manual, when the servo head is within a designated track location, an offset forward or reverse signal may be applied by the control electronics to the servo. The latter respective signals are negative or positive D.C. voltage steps injected in the servo loop, which serve to move the servo head and, thus, the entire head assembly slightly in either direction with respect to the center of the servo track in order to maximize the signal obtained from the recorded data tracks with respect to the playback heads.
In the above-indicated prior art magnetic disk recorders the gain of the servo amplifier has been selected relatively low to minimize servo oscillations, overshoot or undershoot in response to the D.C. offset forward or reverse signal.
However, when it becomes necessary to substantially increase, for example to double, the recording track density with respect to these prior art magnetic disk recorders, the above-indicated servo gain cannot further satisfy the consequent requirement for an improved positioning accuracy, as well as shorter response time and, thus, extended servo performance.
One embodiment of the present invention overcomes the above-described disadvantages of prior art positioning servo circuits by providing a reactive filter circuit which allows for increasing the servo loop gain and bandwidth, in combination with a circuit for compensation of overshoot and undershoot transients and delay in response to D.C. signal level changes applied to the reactive filter circuit, as it will be disclosed in the specification in more detail.
Examples of further prior art devices in which undersirable transient overshoot, undershoot and delay in reactive filter circuits occurring in response to sudden D.C. signal level changes may exceed the limits set for reliable operation, include, for example, magnetic tape drive capstan servo circuits, frequency synthesizers, regulated power supply circuits, and many other applications. As it will become apparent from the following specification, various alternative embodiments in accordance with the present invention eliminate the above-indicated disadvantages of the respective prior art devices.
It is noted that throughout the specification and claims under "reactive filter", "reactive filter network", "reactive filter circuit", and like terms, a passive or active filter is understood comprising one or more reactive elements which may be combined with resistive or other filter elements and having at least one filter element coupled between an input, an output and a signal return line of the filter.